The present invention relates to fibrids and their production. In particular, the present invention relates to a process and apparatus for the production of fibrids.
Various processes and apparatus are known which permit the direct production of fibrids. One typical process involves the technique of abruptly expanding a solution of molten polymer by passing it through a suitable orifice. An example of this technique can be found in U.S. Pat. No. 4,332,749. Other examples of known techniques for producing fibrids include that disclosed in Belgian Pat. No. 824,531 in the name of Solvay and Cie., which proposes to produce fibrids by subjecting, to abrupt expansion, a solution containing a polyolefin, which has been subjected to a prior oxidation treatment, and a polar monomer which can be grafted onto the polyolefin. This process involves carrying out two successive steps, which complicates the process and increases the cost and price of the fibrids produced.
In order to avoid resorting to a process involving two successive steps, Belgian Pat. No. 847,491, in the name of Solvay and Cie., proposes to incorporate, in the molten polymer/solvent mixture subjected to the abrupt expansion, a polar monomer which can be grafted onto the polymer used, and to cause the grafting onto the polymer before the abrupt expansion. However, this process exhibits another serious disadvantage. In fact, a secondary reaction in which the polar monomer is grafted onto the solvent causes the formation of oligomers, consisting of grafted solvent, which oligomers are subsequently found in the water in which the produced fibrids are suspended, and the hydrolysis of which leads to corrosive products. This reaction also causes the loss of some of the polar monomer and solvent. These processes also exhibit a common disadvantage, namely that the grafted polyolefin is grafted throughout its bulk, whereas, in order to ensure an adequate compatibility between the polyolefin fibrids and the cellulose pulp in the case of suspending the fibrids in water, it suffices for the fibrids to be grafted on their surface. The known processes thus involve the use of a large excess of polar monomer, relative to the amount which is actually necessary.
Another conventional process for producing fibrids is disclosed in U.S. Pat. No. 2,988,782. This process involves a solution of a wholly synthetic polymer which is added to a precipitant for the polymer under shear conditions such that the system has a precipitation number of at least 100. However, this process requires some rather complex calculations to determine the precipitating number. Further, this process is limited to polymers for which a solvent and solvent miscible precipitant can be found.
Furthermore, these processes for the production of fibrids require the use of organic solvents and/or organic solvent/non-solvent mixtures. The recovery, storage and disposal of these organic solvents constitutes a major cost.
Fibrids may also be formed by melt blowing. In the conventional melt blowing process, polymer is melted, forced through a spinnerette, and drawn to a fine fiber diameter by a blast of hot air at a temperature ranging from about 500.degree. to 600.degree. F. It is necessary to chill the hot, fine diameter fiber immediately after it is formed so that it does not shrink back to a larger diameter fiber or a ball of melted polymer. In an attempt to alleviate the situation, current technology employs the use of a water spray to cool the melted polymer strands. However, it is impossible during the melt-blowing process to achieve sufficient cooling to optimize the fibrid yield.
In fact, none of these processes describe an adequate method for cooling fibrids and maximizing the fibrid yield. Thus there remains a need for a simple, inexpensive and efficient method for manufacturing fibrids which advantageously does not involve the use and recovery of organic solvents, yet which provides for optimum fibrid yield.